1. Field of the Invention
The present invention relates generally to spread spectrum communications between two communications devices, and more particularly, to a method and system for maintaining synchronization between such communications devices in a power-conserving mode.
2. Background of the Invention
Nowadays spread spectrum communications that employ frequency hopping are widely used in a variety of communications systems for communicating between two or more communication devices, such as between a base unit and a portable unit (e.g., handset or headset) of a cordless telephone system. To provide proper communication, the operations of such units must be time-synchronized to each other.
For example, with reference to FIG. 1, cordless telephone system 100 includes base unit 10 and one or more portable units 20. Base unit 10 is a landline telephone base station associated with telephone line 30 of Public Switch Telephone Network (PSTN) 40. Portable unit 20 is a remote unit that communicates with base unit 10 via known wireless communications technology. To allow communication between portable unit 20 and base unit 10, portable unit 20 and base unit 10 each have radio transmitters 21, 11, receivers 22, 12, and antennas 23, 13, respectively, to exchange signals. Typically, portable unit 20 includes rechargeable battery 24 that is charged when portable unit 20 is placed in cradle 15 located in base unit 10. Base unit 10 is powered by current from an alternating current (“AC”) outlet (e.g., of a house, office, etc.). Cradle 15 need not be co-located with base unit 10. In models in which base unit 10 has no cradle, a separate cradle unit can be used to charge the battery of portable unit 20.
In operation, receiver 22 of portable unit 20 is normally kept in a full-power mode when using telephone system 100 to originate and receive calls. When a user wishes to place a call from portable unit 20 and dials a telephone number of a called party, transmitter 21 of portable unit 20 sends a signal to base unit 10. Upon receiving the signal from portable unit 20, base unit 10 seizes telephone line 30, through which the user can communicate with the called party. Similarly, when base unit 10 receives an incoming call, base unit 10 transmits an appropriate ringing signal to portable unit 20, thereby activating a ringer in portable unit 20.
As battery 24 provides limited power, the period of time for reliable operation of portable unit 20 is limited. To extend the reliable operating time, it is required that portable unit 20 be able to go into a low-power or standby mode with minimal power consumption when it is not in use. In some cases, portable unit 20 further goes into a sleep mode after a period of time during the lower-power mode and wakes up periodically for detecting any incoming signals. If no signal is detected, portable unit 20 goes back to the sleeping mode until next cycle is started or when any incoming signals are detected.
It is also necessary for the link between portable unit 20 and base unit 10 be re-established promptly when there is a stimulus at either end of the link. An example of such a stimulus on portable unit 20 side would be a telephone keypress. An example on base unit 10 side would be an incoming call (i.e., ringing). It is also required that if the link is disrupted in the low power mode, for example, when changing battery 24 of portable unit 20, this condition be detected at both ends of the link, and that base unit 10 and portable unit 20 perform a link search to re-establish the link.
Conventionally, cordless telephone system 100 initiates communications over one of multiple channels and continues to use the same channel for ongoing communications between portable unit 20 and base unit 10. This type of cordless telephone system employs fixed-frequency RF (radio frequency) link between portable unit 20 and base unit 10. With recent rulings promulgated by the Federal Communications Commission, spread spectrum systems, including frequency hopping systems, are also used in some cordless telephone system. A frequency hopping system changes the frequency of the transmission many times in a second in accordance with a pseudo randomly generated list of channels. The frequency hopping system allows for an increased spectral utilization.
It, however, causes problems if simply using the old architecture to employ the frequency hopping system. Most notably, frequency hopping links are sensitive to timing differences. For a fixed-channel link, a transmission on one side of the link is always received by the RF link on the other side. With a frequency hopping link, as the channel changes with time, a unit expecting to receive a message from its partner must be tuned to the same channel on which its partner is transmitting to enable receipt of the transmission.
In addition, the method used to implement the RF standby mode should be compliant with any relevant regulations for frequency-hopping devices, and should be robust even if several of the RF channels in the hop sequence are unusable due to interference.